Hitherto, there have been known various synthetic resin packaging containers for cooking by a microwave oven, which are sealed up after the packaging containers contain sterile cooked rice, retort food, and frozen food, or the like, and heated in the microwave oven to cook the contained products at the time of eating. However, when the packaging container is heated in the microwave oven, inside pressure of the packaging container is increased by steam and the like generated from the food contained therein, the packaging container explodes, and the food is scattered. As a result, the inside of the microwave oven becomes a mess, and harm such as a scald may be inflicted on the human body.
Therefore, before the above packaging container is heated for cooking in the microwave oven, the packaging container is partially unsealed or a hole is produced in the packaging container to discharge steam and the like, which are generated in the packaging container, to the outside thereof so as to prevent the explosion of the packaging container.
However, the above-mentioned method requires care to an ordinary consumer. Because steam generated from the food inside the packaging container by heating in the microwave oven is discharged to the outside of the packaging container immediately, the packaging container has a disadvantage in that the effect of heating and steaming the contents of the container by steam lowers, and the taste of the contents become worse.
To eliminate the disadvantage described above, various packaging containers for cooking by a microwave oven have been proposed, in which when a container made of a synthetic resin having a flange part is filled with food or the like, and a peripheral part of a lid is heat-sealed to the flange part, a projecting part that projects in an inside direction of the container is formed at the peripheral seal part.
These packaging containers are prevented from exploding by allowing the projecting part to spontaneously open due to the increase in an internal pressure in the container at heating/cooking by a microwave oven. (For example, see Patent Documents 1-3)    Patent Document 1: JP 62-235080 A    Patent Document 2: JP 11-171261 A    Patent Document 3: JP 2000-62858 A
However, in the packaging containers for cooking by a microwave oven described in these Patent Documents, the peripheral seal part of the flange part and the lid projects in an inside direction of the container. Therefore, it is not easy to form the projecting part that spontaneously opens during heating on a practical level on the flange part with a limited space. More specifically, the projecting part may be displaced to impair the sealing performance of the peripheral seal part, or the projecting part may explode without spontaneously opening during heating. Further, there has been a problem in that the flange part needs to be configured with a large width, which increases the amount of materials constituting the container, resulting an increase in cost.